Just Before Dawn
(1981)

Young adults go camping and encounter scary inbred residents. 

Campers start dying off until the Park Ranger (George Kennedy) shoots the killer. 

The last of the campers are attacked again.... by the killer's twin. 

Connie (the shy camper who has evolved from dowdy to sexpot) goes psycho and rams her fist down the killers throat causing him to die, just before dawn.

Thanks Don P!